The coordination chemistry of the pentadentate 2,2,6,6-tetrakis(aminomethyl)-4-azaheptane (ditame)

Abstract

The protonation and metal complex formation of the branched pentaamine ligand 2,2,6,6-tetrakis(aminomethyl)-4-azaheptane (ditame, L) with CoII, CoIII, NII, CuII, ZnII, and CDII have been studied. The crystal structures of [Ni(ditame)(EtOH)] Br2, [CO2(ditame)2(O2)Cl4 ·4H2O, and [Co2-(ditame)2(O2)]Cl2 [ZnCl4][ZnCl3(H2O)]·H2O were determined by X-ray diffraction analyses. The metal complexes showed octahedral coordination geometries with exclusive formation of six-membered chelate rings, the four primary amino groups being in equatorial positions and the secondary amino group (Nsec) apical. This geometry enforces a relatively short M-Nsec bond. Complex formation in aqueous solution was evaluated by potentiometric titration. The ditame ligand forms a surprisingly stable NiII complex with logKNiL = 17.8 (25 °C, 0.1 M KNO3), and formation of protonated species [Ni(Hnditame)](n+2)+ (n = 1, 2) is not significant. Different behavior is evident for ZnII and CdII, with which protonated species are extensively formed, while the stability of [ML]2+ is relatively low. Complex formation between ditame and CuII in the range 3 < pH < 7 resulted in the formation of [Cu(H2ditame)]4+. In slightly alkaline solution, the dinuclear [Cu2(ditame)2]4+ appears to be formed. The structural assignment of this dimer is based on its UV/Vis and EPR characteristics, with the EPR data indicating a CuII···CuII separation of about 5.9 Å. The redox behavior of the Ni and Co complexes was investigated by cyclic voltammetry. The mononuclear species showed quasi-reversible electron transfer for the NiII/III and ColII/III couples. For the dinuclear peroxo/superoxo-bridged CoII/III complexes, quasi-reversible electron transfer corresponding to oxidation/reduction of the O22-/O2- bridge was observed. ( Show more